Sodium 5-nitrotetrazolate (“NaNT,” 6) has found application as a reactant or constituent in a variety of explosives and propellants and additional uses are contemplated. Typically, NaNT is synthesized via a Sandmeyer type reaction that involves displacement of a diazonium group by a nucleophile, in this case nitrite ion resulting in a nitro group, in the presence of cupric salts. C. Galli, “Substituent Effects on the Sandmeyer reaction. Quantitative Evidence for Rate-determining Electron Transfer,” J. Chem. Soc. Perkin Trans. II, No. 5, 1984, pp. 897-902; U.S. Pat. No. 4,093,623. Energetics chemists have been utilizing this method for a number of years to produce NaNT in small batches.
This procedure, outlined in FIG. 1, involves addition of a solution of commercially available 5-aminotetrazole (“5-AT,” 1) in aqueous nitric acid to a solution of copper(II) sulfate and sodium nitrite to generate the diazonium ion (3), which undergoes substitution to afford the acid copper salt of 5-nitrotetrazole (“5-NT,” 5). During the addition of the 5-AT and nitric acid, the reaction temperature must be tightly controlled at or below 18° C. due to the thermal instability of the diazonium intermediate. The second process step utilizes aqueous sodium hydroxide to convert the acid copper salt of 5-NT into NaNT and generates copper oxide as a byproduct.
This method is problematic, particularly during larger scale procedures, due to “micro-detonations,” which occur if the mixing of the 5-AT and sodium nitrite solutions is not tightly controlled. These micro-detonations may be caused by nitrogen oxide fumes from the reaction solution reacting with droplets of 5-AT on surfaces in the reactor to form 5-diazotetrazole (4), which may spontaneously detonate in solution when the concentration exceeds 1%.
These micro-detonations may be strong enough to break glass and may result in release of the potentially explosive reaction mixture. It was determined that inclusion of a small amount of CuSO4 to the 5-AT solution prior to addition to the CuSO4-nitrite solution was effective in preventing the micro-detonations by catalyzing conversion of 5-diazotetrazole, in the presence of nitrite, to 5-NT. Use of these cupric salts, however, adds additional steps (and cost and/or time) to the procedure, which results in lower overall reaction yields. These additional operations include two manual filtration steps in which operators are exposed to considerable quantities of CuH(5-NT)3 and NaNT, both of which are explosives. In considering this process, it is quite clear that a less hazardous, alternate procedure is needed for large scale laboratory production of NaNT.
As opposed to batch processes, such as those described in U.S. Pat. Nos. 3,054,800 and 3,111,524, this invention provides a simple, continuous flow process for the synthesis of 5-nitrotetrazolates starting from 5-AT, which is converted directly, via an ambient temperature Sandmeyer reaction, to a salt of 5-nitrotetrazolate without the use of copper, as illustrated in FIG. 4.
U.S. Pat. No. 7,253,288 to R. N. Renz, M. D. Williams, and J. W. Fronabarger, also describes an alternate method for producing NaNT utilizing microreactor technology, which does not use copper to stabilize the tetrazole diazonium intermediate and which involves direct reaction of 5-AT/nitric acid with sodium nitrite at ambient temperature in a continuous flow regime. Unlike a batch process, this procedure generates only very small amounts of the unstable reaction intermediates in a dilute media, which are subsequently consumed via substitution as a part of the flow process. This process provides a safe method for preparation of 5-nitrotetrazolates, as only minor amounts of the intermediates are generated per unit time and accumulation is not possible, but requires extensive time and an appropriate microreactor system optimized for 5-NT production for the flow process.
U.S. Publication No. 2014/0206885 also describes a continuous flow process for production of 5-nitrotetrazolates, which does not use copper to stabilize the diazonium intermediate reactant and involves direct reaction of 5-AT/nitric acid with sodium nitrite at an elevated reaction temperature to promote expedient substitution of the nitro group in place of the diazo group and to ensure rapid consumption of hazardous reactants. However, because the diazonium intermediate becomes unstable at higher reaction temperatures, instead of being quickly converted into the desired 5-NT, the diazonium intermediate may in certain infrequent cases instead form minor amounts of 5-azidotetrazole, which is a highly explosive and undesirable side product. As a result, the final product may have an undesirable purity profile and additional purification steps may be required.
The methods for preparation of 5-nitrotetrazolate salts outlined above may be prohibitive either in terms of time and safety for the batch process and/or for possessing an appropriate microreactor system optimized for 5-NT production for the flow process. There is a need to improve the efficiency and safety of the chemical process by providing a method for preparation of 5-nitrotetrazolate salts, specifically NaNT, quickly from 5-AT and utilizing a method in which all of the unstable intermediates are quickly and fully consumed at ambient temperatures.